A Nasa spacecraft set for launch early next month will explore the two biggest asteroids in the Solar System.

Asteroids are believed to be the building blocks of planets - primordial relics left over from the formation of the Solar System 4.6 billion years ago.

The Dawn mission will launch from Cape Canaveral Air Force Station, Florida, on 7 July, on a mission to study the asteroids Ceres and Vesta.

Dawn will reach Vesta in 2011 before going on to visit Ceres in 2015.

We're going back in time to the early Solar System

Christopher Russell, UCLA

"Ceres and Vesta have been altered much less than other bodies," said Christopher Russell, the Dawn mission's chief scientist.

"The Earth is changing all the time; the Earth hides its history, but we believe that Ceres and Vesta, formed more than 4.6 billion years ago, have preserved their early record."

CERES

Biggest object in the asteroid belt

930km (580 miles) across

Discovered in 1801 by Giuseppe Piazzi

Icy layer beneath dusty surface

Ceres is almost spherical and is thought to harbour a layer of water ice some 60 to 120km (40 to 80 miles) thick beneath its rocky surface.

At a meeting of the International Astronomical Union (IAU) last year, Ceres was elevated in status from merely the biggest body in the asteroid belt, to a "dwarf planet" - the same designation now held by Pluto.

While Ceres is a "wet" object, Vesta is devoid of water and appears to have been resurfaced by ancient lava flows.

Dawn will travel to the asteroid belt to carry out a detailed study of their structure and composition, shedding light on their evolution and the conditions in which these objects formed.

The mission's objectives include:

study internal structure and density

determine size, composition, shape and mass

examine surface features and craters

understand the role of water in controlling asteroid evolution

Dawn's instruments include a gamma ray and neutron spectrometer that can detect the hydrogen from water.

Evidence of whether water still exists on Ceres could come from frost or vapour on the surface. There may even be liquid water under the surface.

The water is thought to have kept Ceres cool throughout its evolution. By contrast, Vesta was hot, melted internally and became volcanic early in its development.

Frozen in time

While Ceres remains closer to the ancient state, Vesta evolved further over its first few millions of years of existence.

Dawn is expected to send back high-resolution images of these worlds, including, perhaps, mountains, canyons, craters and ancient lava flows.

VESTA

525km (326 miles) across

Surface has distinctive light and dark areas

Discovered in 1807 by Heinrich Wilhelm Olbers

Pieces of Vesta have fallen to Earth as meteorites

The instruments will help identify minerals on the surface and the elements they contain.

"[Ceres and Vesta] are revealing information that was frozen into their ancient surfaces," said Professor Russell, a professor at the University of California, Los Angeles (UCLA).

"By looking at the surface and how it was modified by the bombardment of meteoroids, we will get an idea of what the early conditions of Ceres and Vesta were and how they changed.

"So Dawn is a history trip, too. We're going back in time to the early Solar System."

Dawn is scheduled to fly past Mars by April 2009, and after more than four years of travel, the spacecraft will rendezvous with Vesta in 2011.

The spacecraft will orbit Vesta for about nine months, before setting off in 2012 for a three-year cruise to Ceres.

Dawn will rendezvous with its second target in 2015, to conduct studies for at least five months.